Add insert_at operation for inserting into a list at a specific index

[nathanjmcdougall]

Problem

There is currently no way to insert an item at a specific position in a YAML sequence while preserving comments on existing items. The only options are:

1. �ppend — adds at the end only

eplace the entire list — loses all internal comments

For example, inserting a pre-commit repo at index 1 (between two existing repos) currently requires replacing the entire
epos list, which destroys any comments within the existing items.

Proposal

Add an \insert_at\ method to \Document\ (and \Editor):

\\python
doc.insert_at('repos', index=1, value={'repo': 'https://...', 'hooks': [...]})
\\

This would require a new \Op\ variant in the Rust layer (sibling to \�ppend):

\
ust
Op::InsertAt { index: usize, value: Value }
\\

The patcher would locate the byte position of the Nth item and insert the new content before it, matching the indentation of its siblings.

Use case

In usethis, pre-commit hooks have a defined ordering. When adding a new hook repo, it often needs to be inserted between existing repos rather than appended at the end. Without \insert_at, the workaround is a full list replacement via
eplace('repos', value=new_list), which loses all comments within the list.

Notes

• Comments on existing items would be fully preserved since they are not touched.
• The new item's indentation should match existing siblings (related to indentation inference in the patcher).